Baptism
It is the first sacrament. God makes you his child, washes away sin, and makes you part of the Church.
The sign
Water, and the words: "I baptize you in the name of the Father, and of the Son, and of the Holy Spirit."
Who gives it
Normally, a bishop, a priest, or a deacon. In case of need, such as danger of death, anyone can baptize. To do so, the person must want to do what the Church does, and use water and those words.
How many times
Only once in a lifetime.
What the Bible says
Matthew 28:19
Going therefore, teach ye all nations; baptizing them in the name of the Father, and of the Son, and of the Holy Ghost.
Douay-Rheims Bible (Challoner revision), public domain.
Source: Catechism 1213, 1239-1240, 1256, and 1272